In recent years, high power modules as inverters for an electric motor have been developed in the field of industrial equipment. Various improvements have been proposed for the purpose of raising reliability, based on a conventional structure wherein a ceramic circuit substrate having a chip mounted thereon and having a high thermal conductivity is soldered to a copper base.
For example, there has been proposed a structure, wherein an Al—SiC composite is used as a base material, and the Al—SiC composite is soldered to an aluminum nitride substrate with an Al-circuit (hereinbelow, referred to as the Al—SiC composite/aluminum nitride substrate with an Al-circuit).
This proposed structure can maintain practical properties even after thermal stress is repeated in 3000 cycles. This structure has significantly reliability in comparison with a conventional copper base/aluminum nitride substrate with a copper circuit. However, the proposed structure has not been widely used since this structure creates, e.g., problems in that a chip needs to have a larger size because the proposed structure has a greater thermal resistance than the conventional structure, and that the module cost is raised because the Al—SiC composite is expensive.
Under the circumstances, further investigation and development have been made for the purpose of raising reliability. In the investigation and development, there is proposed an idea wherein when producing an Al—SiC composite, the Al—SiC composite is made integrally with a ceramic substrate or a ceramic circuit substrate.
This idea has two advantages. One is that it is possible to decrease the thermal resistance since no solder is used. The other is that it is possible to eliminate a step for bonding a metallic material to a ceramic material.
However, even if being produced according to this idea, a conventional circuit substrate has a structure that it is quite difficult to be produced or there is created a problem in mass production since, e.g., a baseplate and a ceramic substrate need to be provided with complicated shapes each other when being bonded. In a case wherein the conventional circuit substrate is supposed to be quite easy to be produced, there has been created a problem that the circuit board cannot ensure high reliability since the circuit substrate is subjected to thermal hysteresis in practical use to have heat radiation properties and electric properties easily deteriorated.
The inventors have made various experiments and investigations, taking the circumstances stated earlier into account. The inventors have attained the present invention based on the finding that an integral ceramic circuit board, which is resistant to repeated thermal stress and is therefore superior in reliability, can be produced, amazingly, only by using, as a material, a layered structure wherein a specific metal layer is layered on a ceramic substrate and a porous molded product made of silicon carbide in a specific arrangement, and applying a conventional Al—SiC composite producing step to the layered structure.